HV transformers

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
User avatar
Richard Hull
Moderator
Posts: 12281
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

HV transformers

Post by Richard Hull » Fri Jun 21, 2002 4:03 pm

The high voltage transformer is often a specialized bird. I will try and list a few below with pro's and con's regarding their use. I will limit the discussion to 60hz mains type of transformers.
*************************************

Neon Sign transformers:

These are a fairly inexpensive solution (used) to creating a basic HV supply. They are available in voltages from about 3kv to 15 kv. (check your yellow pages for neon sign shops where you might obtain a used unit) Most all neon trannys above 5 kv are center tapped, case grounded units. This, severly limits there use for really high voltages. They are however ideal for demo fusor work where normal voltages are in the 2-8kv range.

All neon sign transformers are magnetically shunted type transformers. This means that they automatically, under load, reduce their nameplate voltage so that at the stated current on the name plate, they will only produce about 700 volts or less of output! There are two sub types of the neon sign transsformer based on current. They are 30 ma and 60 ma transformers. You should only try and get the 60ma units.

As a shunted transformer is loaded down, (plasma glow mode begins), the high voltage drops with ever increasing current so that a 5kv, 60ma transformer might only produce a maximum of 4 kv at 20 ma, 3 kv at 35ma, 2 kv at 45 ma, etc.

This characteristidc is undesirable in a beefy power supply, but quite acceptable in a small demo system and acts to self limit the system and prevent instant inner grid melting by the novice.

The best way to utilize these transformers is to make a full wave rectifier design. The center tap or case is grounded. The two output insulator knobs are each connected to the cathode of 12kv, 500ma, microwave diodes that have their anodes tied together. This tie point is now a full wave rectified high voltage NEGATIVE hot DC lead with the case and ground being the positive return. A 15kv 60 ma neon sign transformer can easily put out a respectable negative 8-9 kvdc in this manner. (remember that you will only get about half voltage in this configuration.

************************************
Plate Transofmrers:

These include many types of X-ray transformers as well. These represent one of the best types for the serious high power amateur fusorite. They are usually found used at hamfests or on e-bay. It is important to know the rating which is often found on the nameplate. Note** three phase transformers are not a good bet and you would not be able to achive full rating with them. Make sure that your tranformer is single phase, 60 hz, and has a 120 volt or 240 volt type primary.

These transformers are not the type to plug into a wall outlet as the fusing or breaker for a common wall outlet is not going to be able to handle the surge current and might trip instantly. For the really serious fusor buffs a 20amp or greater special drop or outlet is needed with both 120/240 capability. (much like a range or dryer outlet.)

Needless to say that not only the wall outlet power but the transformer secondary high voltage on these systems is totally unforgiving and you will not get arcs, but electrical explosions if wiring errors or faults occur.

.... This business is as serious as a heart attack.....

KNOW WHAT YOU ARE ABOUT...!!!

These plate type transformers weigh in at a minimum of 50 lbs and go up from there. Some are in oil and some are dry with heavy cast insulation.


********************
Variac control

All transformers will benefit from slowly raising their primary input voltage from zero to the full outlet voltage. This is best accomplished with a special variable transformer called a variac and these are obtained mostly at hamfests. Make sure the variac can handle the voltage and current of your transformer. A side benefit is that the secondary voltage also rises slowly, and even a simple rectifier system will allow a nice, smoothly variable output from zero volts to the maximum high voltage on the secondary of the trasnformer.

A super simple small, variable, supply would only need 4 components. Variac, transformer, and two diodes.

**************************
Potential Transformers:

These are very rare birds, indeed! They are outrageously expensive new, but represent the finest of all worlds. They are used in electrical switch yards for metering high voltages and can often handle 2-3 kilowatts output. A light one weighs 80 lbs. They range in voltages from 2400 volts to 80,000 volts. Tesla coilers love 'em too and are paying small fortunes for them on e-bay when they are offered.

I could not think of a more attractive transformer than a potential transformer for fusor use. They are rugged, sturdy and can take a bit of abuse.

******************************
Pole or distribution tranformers:

Just say no!



Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

guest

Re: HV transformers

Post by guest » Sat Jun 22, 2002 12:29 am

Why "no" on the pole pigs ??? They are the most cost effective of all and more easy to get than an x-ray unit. Not to mention their bullet-proof durability.

Mark Rowley
Garage Scientist(unlicensed)

guest

Re: HV transformers

Post by guest » Sun Jun 23, 2002 5:14 pm

Pole Pigs have a tendancy to blow apart if accidentally shorted..... They tend to throw flaming oil in all dirrections. When I was about 14, a kid threw a coat hanger on a pole pig... not only did it blow ....it severely burned the idiot kid. Just imagine if your grid melted and touched the case and at .5 amp it surely would in less than a minute.... snap... crackle... boom with the force of a stick of dynamite.

Larry Leins
Physics Teacher

guest

Re: HV transformers

Post by guest » Mon Jun 24, 2002 4:58 am

Maybe we all forgot, but there is such a thing called a fuse. Or maybe a current limiter...etc...etc...etc

Pole pigs are good, Good, GOOD !!!!!

I shutter to think ANYONE would operate an LDT in a non-fused full current mode. That is the epitome of stupidity.

Mark Rowley
Garage Scientist(unlicensed, but SAFE)

guest

Re: HV transformers

Post by guest » Mon Jun 24, 2002 1:33 pm

Mark:

A lot of our surfing buddies out there are voltage virgins. You have to spell it out for them. Fuses would probably would take out the hazard. But the setup of a mini substation would take conciderable safety measures. I plan to enclose my capacitor bank and high voltage behind a wire mesh fence. Besides the public out there has to be protected from their own ignorance. Since the word has gotten out , I have at least one or two visitors a day gawking at this stuff.
I have to watch them like a hawk.
Irregardless of age all untrained people need to be treated as two year olds.

Ebay has put potentually lethal stuff into the general public.

Larry Leins
Physics Teacher

User avatar
Richard Hull
Moderator
Posts: 12281
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: HV transformers

Post by Richard Hull » Mon Jun 24, 2002 5:08 pm

Larry is correct. I have owned and used about 6 pole pigs over the years. (12KW Tesla systems) They are great if you absolutely must have 10 kilowatts at 12,000 volts on immediate demand. They are bitches to control and get out of control easily even in competent hands. Remember that by using a pole pig in reverse, you have brought the high tension power lines right into your home!!

NO ONE is working on a Fusor that needs 10KW continuous draw! Therefore, a pole transofrmer dialed back to 300-800 watts is a ridiculous was of energy and near impossibility to control. My fusor III at its peak power usage and it could NOT be there more than 20 seconds drew 500 watts. 35,000 volts @ 14ma, continuous.

So, 1. Most pole pigs can't appraoch the voltages we need. 2. Vertually no person here has the savey to dial one back to an idle of 1/20th capacity. 3. Such a transformer in the hands of HV and power neophytes would be like handing out sticks of dynamite to kids on the 4th of July.

Folks here have enough to worry about with the multiplicity of disciplines touched upon by the attempt at a fusor without having to walk into a power control minefield by being told that a pole pig will solve all their power problems. Quite the contrary, it will most likely be the beginning of problems and expense to no end.

I say again.....Do not acquire or use a pole tranformer on a fusor project for any reason!!! I have 'em on hand here, know how to dial 'em back after 12 years of working with them, and WILL NOT USE 'EM for any of my fusor work.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

guest

Re: HV transformers

Post by guest » Tue Jun 25, 2002 3:42 am

I agree. For Fusor work, there are much better things to use than a pig. However, that only applies to someone who is(and will always be) involved in such work. Like most of us here, the Fusor is just one of many projects to be worked upon. As a result, one would want a well rounded shop which is capable of many demands. A jeweler need jewlers tools, an electrician needs electrical tools, a policeman needs his and so does a teacher.

My past projects could have used one and my future ones require it. Although the fusor does not.

When I made my second reply to this string, I suppose that I was a bit reactionary. I apologize. At the time I saw it as an attack on the Pole Pig for any type of use whatsoever.

Mark Rowley
Garage Scientist(unlicensed)

DaveC
Posts: 2346
Joined: Sat Jun 30, 2001 5:13 am
Real name:

Re: HV transformers

Post by DaveC » Tue Jun 25, 2002 9:03 am

I would tend to agree with all the cautions here about the used Utility Trsf. The main cause of the instability that Richard spoke about is the large inductance.....
(Henries !!). Combined with power supply filter capacitors, a condition called ferro-resonance occurs which can produce very large voltages, way larger than the simple turns ratio might suggest.

The use of Circuit breakers, AND fast fuses is highly recommended. Many high current fuses will take 100 percent overloads for virtually forever... and it may take from 4 to 10 X rated current to get it to open in a few seconds.n With a big transformer.. you can get extremely large amount of stored enegry...before the protective gear tries to shut down... and then when circuit breakers and fuses try to clear a fault condition, they are instantly arced over and the system is on its when to being a cinder...

One good practice is to put current limiting resistance in the output of your high voltage DC source. In the Xray industry, 1ohm per volt of output is a common current limiting resistor value... This keeps all surge currents in the few amps max range.

Having said all that... A distribution transformer is a gold mine of wire, coils and core.. A 10 kva unit is a lil bitty thang anymore to utility folks, so good one might not be too hard to come by. But ... you have a few gallons of transformer oil to handle and more than a few pounds of core, case and windings.

Dave Cooper

User avatar
Richard Hull
Moderator
Posts: 12281
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: HV transformers

Post by Richard Hull » Tue Jun 25, 2002 1:50 pm

The bottom line is......If you are fully capable of safely controlling such a leviathan AND if you have one of these 300 lb plus monters loitering around the house AND don't want to pay or search for a smaller solution,....fine. Use it.

If you don't own one or if you can't imagine the expense or hassle of pulling a 60 amp or greater 240 volt quick disconnect box and drop to you lab area or foresee 10kw electrical project in the future. Never look at or consider the pole transformer as an option.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

guest

Re: HV transformers

Post by guest » Tue Jun 25, 2002 2:19 pm

Rewinding transformers is a tricky and expensive business to set up... it costs tens of thousands of dollars. Most people who do this stuff
usually apprenticeship in the Navy or for an existing shop. I have worked in such a place for a summer... EE's are considered as idiots until they have a year of actual experience under their belt. A skilled transformer winder was paid 20.00 Dollars an hour in 1976...
they all belong to the Electrical Workers Union. The work is so demanding and tedious that it is alot like traffic controller at an airport. It is very serious business.

Larry Leins
Physics Teacher

Post Reply